Which description best defines an unsaturated hydrocarbon?

• A. Contains a carbon-carbon double bond
• B. Contains only single bonds
• C. Contains carbon-carbon triple bonds
• D. Contains no hydrogen

Answer: (A)

Unsaturated hydrocarbons have fewer hydrogens than the corresponding alkane because they include multiple bonds between carbon atoms, which changes how many hydrogens the molecule can hold. The most straightforward way to identify this is the presence of a carbon–carbon double bond, since an alkene has at least one C=C bond and that double bond is the reason for the hydrogen deficiency compared to alkanes. While a carbon–carbon triple bond also means the molecule is unsaturated, not all unsaturated hydrocarbons have triple bonds, so using the double-bond feature as the descriptor covers the common cases best. The idea that a molecule contains only single bonds describes a saturated hydrocarbon, and the notion of having no hydrogen is not a general or reliable indicator of unsaturation.